Improvements in hole mobility have been recognized in PMOS devices (i.e., metal-oxide-semiconductor (MOS) devices with n-type, or n-doped, substrates and p-type charge, or hole, carriers), or transistors, by compressively straining the channels of such devices. Similar improvements in electron mobility have been realized in NMOS devices (i.e., MOS devices with p-type, or p-doped, substrates and n-type charge, or electron, carriers), or transistors, when the channels thereof experience tensile strain Enhanced carrier mobility may be used as a means to improve transistor speed and performance.
A number of techniques have been researched to compressively stress PMOS regions, including the inclusion of a strained layer of silicon-germanium (SiGe) within the source/drain regions to compressively stress the channel region between the source and drain, which results in an increase in hole mobility of up to 50%. Strained silicon layers have also been fabricated on relaxed silicon-germanium layers in the channel regions of NMOS devices to create uniaxial tensile stress in the channel region between the source and drain to enhance electron mobility and increase the speed of NMOS transistors.
When silicon-germanium is used, a layer of silicon-germanium is formed, typically by ultra-high vacuum chemical vapor deposition (CVD) techniques. The layer of silicon-germanium is then capped with a thinner silicon film. Lattice mismatches between the relaxed silicon-germanium layer and the silicon capping layer generate the desired tensile stress. Unfortunately, the ultra-high vacuum CVD techniques that have been used to fabricate silicon-germanium layers are extremely expensive and, thus, less conducive to use in large-scale semiconductor device fabrication processes.
A number of other experimental techniques for stressing channel regions of semiconductor device structures have also been developed. Tensile strain has been generated in the channels of NMOS devices by using silicon carbide (SiC) in the source/drain regions. Semiconductor device structures have also been bent (which may, e.g., be effected in packaging and encapsulation) to stress the transistor channels. In addition, semiconductor device structures have been fabricated with stress-inducing silicon nitride capping layers.
It would be desirable to develop a process by which transistor channels of semiconductor device structures may be stressed economically and on a scale that is suitable for incorporation into semiconductor device fabrication.